CODEWITHSUNDEEP
javascripttypescript
David Hellsing
David Hellsing

Reputation: 108530

Typescript: Array with arrays fails when using map

type Point = [number, number]
type Points = Array<Point>

const ok: Points = [[0, 0]]
const fail: Points = [0].map(() => [0, 0])

Type 'number[][]' is not assignable to type '[number, number][]'.

Any ideas?

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Upvotes: 1

Views: 207

Answers (1)

Sergiu Paraschiv
Sergiu Paraschiv

Reputation: 10163

It's because of how map is typed.

The "official" definition is:

map<U>(callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): U[];

If you don't specify U then default U[] will be [] (same as Array).

The problem is that [number, number] is a subset of Array AND map without specifying U would mean the return type will be Array.

The two types not being compatible, you get that error.

Solution is to specify what map is allowed to return:

const okToo: Points = [0].map<Point>(() => [0, 0])

On the other hand I'd expect this to fail too:

const notOk: Points = [0].map<Point>(() => [0, 0, 0])

but on TypeScript 2.6.1 it's allowed. It's an error in 3.3 though, which is really nice.

Upvotes: 4

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